Air bearings offer a great advantage over conventional bearings where very high shaft speeds are involved. The shaft rides within the shaft housing on a thin film of fluid, such as air, and the frictional resistance is very slight. This fluid film may be created by having the bearings externally pressurized or the fluid film may be created by the action of the shaft rotating rapidly proximate a series of overlapping foils mounted to the shaft housing. These foils create wedge shaped gaps proximate the shaft and a velocity gradient is established in the boundary layer of air between the shaft and the foils. The viscous or shear forces in this boundary layer draw fluid into the wedge shaped gaps to create the film.
At zero speed there is a relatively high breakaway torque since the shaft rests on the foils in the bottom of the shaft housing. At low speed, there is high sliding friction caused by the rubbing of the shaft on the foils before the hydrodynamic action has taken effect. Also, there is a relatively large clearance between the shaft and the shaft housing in a typical hydrodynamic fluid type bearing since the center line of the shaft moves significantly in a radial direction as the shaft speed increases from zero to a running speed where the center line is substantially concentric with the center line of the shaft housing with only a relatively small, minimum running eccentricity. Devices such as gas turbines do not run efficiently where there is such a large difference between the static and running positions of the shaft center line.
The prior art shows antifriction bearings in series with air bearings as a safety or backup feature. The bearings cannot operate in parallel. For example, Bird U.S. Pat. No. 3,603,654 discloses an antifriction bearing between the shaft housing of the air bearing and a larger support member. A frangible link allows the antifriction bearing to take over in the event that the air bearing seizes. A similar structure produced by General Motors in 1962 had high friction built into the antifriction bearing so that the air bearing was operative almost immediately as the shaft began to turn. Thus, in both structures, the antifriction bearing was only a backup to take over in case of failure of the air bearing.